252 



THE INDIA RUBBER WORLD 



[February 1, 1919. 



and of that existing between this coefficient and the situation of 

 the curve of elongation. 



The addition of chemically indifferent solid fillers has little or 

 no influence on the velocity of vulcanization but does affect the 

 mechanical properties. The elongation at break and breaking 

 load decline and the rubber becomes stifTer as indicated by its 

 elongation curve. The addition of soft fillers markedly reduces 

 the breaking strength and elongation at break. This unfavor- 

 able effect was demonstrated by the presence of ten per cent 

 of paraffine in a test compound. 



The Action of Acceler.ators on the Mech.^nical Properties 

 OF Vulcanized Rubber. 



Rubber vulcanized with powerful accelerators becomes 

 "stiffer," and has less elongation at break than ought to be the 

 case with the vulcanization attained. In other words it does not 

 show a "normal" curve with relation to its vulcanization coeffi- 

 cient. In the case of vulcanization by the use of Yz of one per 

 cent of "accelercne" showing vulcanization coefficient 4.4, the 

 curve occupied about the position of the normal one for a vul- 

 canization coefficient of 7. 



The end point of the curve is situated much higher than would 

 be the case with a "normal" curve similarly located. Also the 

 points of break for normal curves with high vulcanization coeffi- 

 cients are situated very far apart, while for rubbers vulcanized 

 under accelerating conditions they all lie about equally high. 

 From this it follows that rubber vulcanized with an accelerator 

 can be stretched much farther without breaking than that vul- 

 canized under ordinary conditions. With the former, brittle- 

 ness occurs only with a much higher vulcanization coefficient and 

 after reaching a greater degree of stiffness than with the latter. 

 Therefore, whenever accelerators can be used a saving of steam 

 will result and a product be obtained which will withstand 

 greater elongation. 



It is remarkable that rubbers with low viscosity are more 

 accelerated in their vulcanization by similar quantities of ac- 

 celerator than those of a high viscosity. For this reason their 

 mechanical properties deviate more from the normal than those 

 of highly viscous rubbers ; consequently the action of accelerators 

 is of higher importance for rubbers of low viscosity. 



Addition of artificial accelerators under controllable condi- 

 tions is preferable to intentionally leaving in the rubber natural 

 catalysts, the nature and quantity of which are unknown. Ac- 

 cording to the opinion of the Institute, no danger exists of the 

 oxidization of normally treated plantation rubber, packed and 

 stored judiciously, even when stored for long periods. 



Van Rossem hints at the possibility of greatly reducing the 

 variability of plantation rubbers by careful adjustment of the 

 addition of artificial accelerators with respect to the degree of 

 viscosity of the rubber. Care is cautioned here because the ques- 

 tion of proportion of accelerator desirable demands close and 

 accurate study. 



Regarding the possibility of unfavorable influence of accelera- 

 tors in producing "after-vulcanization," it is said that such in- 

 fluence will probably be least for accelerators decomposable at the 

 temperature of vulcanization, such as "acceletene," for example. 

 The Institute advances the opinion that after-treatment of the 

 vulcanized product with retarding catalysts, such as sulphur di- 

 oxide, can become a means of counteracting the harmful influence 

 of after-vulcanization. 



Vulcanization Coefficient as a Measure for the Mechanical 

 Properties. 



With the standard vulcanization of first latex rubbers, the vul- 

 canization coefficient gave an excellent indication of the me- 

 chanical properties. With a definite vulcanization coefficient 

 known, the average course and the end point of the elongation 

 curve could be calculated by using the Schopper machine. 



Comparison of the elongation diagrams of vulcanizations dif- 



fering from the standard method showed that the vulcanization 

 coefficient offers a general indication of the probable course of 

 the elongation curve, but that this is not the case with rubbers 

 containing more than the usual quantities of artificial catalysts. 

 Other factors influence the situation of the end point of the curve. 

 Pre, Uxder, Over and After-Vulcanization. 



In order to determine when a rubber may be pronounced 

 well-vulcanized it is necessary to investigate the resulting prop- 

 erties produced from the inception of the process till it has 

 advanced too far. 



When rubber is heated only a short time with sulphur an 

 alteration takes place which may be considered as the beginning 

 of vulcanization. The resulting product may still be completely 

 dissolved in the usual solvents for rubber. In this case the rub- 

 ber is termed pre-vulcanized. Van Heurn has shown that this 

 prevulcanization is noticeable at ordinary temperature, for after 

 three months the viscosity of a mixture of 92J/2 parts of crepe 

 and 7J4 parts of sulphur appeared to be raised considerably 

 more than could be explained from the recovery of the plas- 

 ticized rubber. Prevulcanization was more marked for a mixture 

 of sulphur, litharge, and magnesia. It is difficult to judge 

 whether any sulphur is combined during this prevulcanizing; 

 however, the fmpression is to that effect. 



Prevulcanization merges into undervulcanization. The latter 

 designation indicates that the product of vulcanization has be- 

 come insoluble though still retaining plastic properties remind- 

 ing one of raw rubber. It should be noticed that undervulcaniza- 

 tion 'is often evident by porosity of the product, developed by 

 steam bubbles in the rubber on blowing off the steam pressure. 



Overvulcanization results in brittleness. Van Heurn points 

 out that it is incorrect to think that where the breaking load 

 attains a maximum, the best vulcanization exists. In practice 

 overvulcanization will be assuredly prevented by not exceeding 

 a vulcanization coefficient of 3.5 at the utmost. 



After-vulcanization takes place in vulcanized rubber stored 

 at ordinary temperature. The increase of combined sulphur is 

 trifling, but at temperatures higher than normal, or when exposed 

 to light, it becomes considerable. A well-vulcanized rubber in 

 which only sulphur is present should possess a vulcanization 

 coefficient of 2 to 4. It is not yet settled how far this result 

 may be altered by the use of fillers and accelerators. 



Alteration of Properties During Vulcanization. 



There exists a gradual change in the mechanical properties 

 when passing from prevulcanization to overvulcanization. A 

 similar continuity of change is observable on the transformation 

 of overvulcanized rubber into ebonite. The quantity of com- 

 bined sulphur also increases continuously coincident with a regu- 

 lar decline of the adhesiveness and solubility. Doubtless this 

 continuity of changes also includes the other properties and 

 applies to both hot and cold vulcanization. 



THE NATURE OF MOTTLING OF VULCANIZED PARA RUBBER. 



A paper by H. Runpel in "Gummi-Zeitung," 1916, page 144, 

 is abstracted as follows in the "Journal of the Society of Chem- 

 ical Industry," October 15, 1918. page S95A: 



The sample examined was free from substitute and yielded 

 only about 3 per cent ash. The lightest patches contained twice 

 as much free sulphur as the darkest. The formation of the 

 patches is explained as follows : After vulcanization the sulphur 

 which has not combined chemically with the rubber passes, on 

 cooling, into the amorphous or the rhombohedral form, except 

 at the surface, where the octahedral crystals quickly separate. 

 Sulphur wanders from the interior of the sample to the surface, 

 and the sample "sulphurs up." At the same time, however, 

 there occurs conversion of the less stable into the more stable 

 form within the sample, the rubber acting as a solvent, and the 

 sulphur wanders from certain parts of the solution and accumu- 

 lates round other centers 



